The previous implementation joined std::env::args() with single spaces,
which silently broke any argument containing whitespace, double quotes,
or trailing backslashes — most importantly manifest paths under
'C:\Program Files' or 'C:\Users\<name with space>'. After elevation
the relaunched process saw a different argv than the original.
quote_command_line_arg follows the standard MSVCRT / CommandLineToArgvW
encoding: quote on space/tab/quote/empty, escape embedded quotes with a
preceding backslash, and double any run of backslashes that would
otherwise be consumed by a following quote (including the closing one).
Tests cover the canonical edge cases plus a round-trip that re-parses
each encoded form with a reference argv parser to confirm fidelity.
Claude
15ba3995a5